Christoph Lehner is a philosopher of physics who has specialized in the history of quantum physics. He has been an adviser and organizer of a series of international conferences on the history of quantum physics.

His contribution to the 2014 Cambridge Companion to Einstein was on "Realism and Einstein's Critique of Quantum Mechanics." It was one of three articles focusing on Einstein's quantum physics. The others were Olivier Darrigol's "The Quantum Enigma" and Roger H. Stuewer's "The Experimental Challenge of Light Quanta."

Lehner's view of Einstein's realism is beyond the simple (though correct) idea that there is a real world of objects "out there" largely independent of human beings and especially physicists. It goes well beyond the logical empiricist view, derived from Immanuel Kant and Ernst Mach that it can logically (and phenomenologically) only consist of the "immediately given" sensations of experience, Kant's phenomenal world and Mach's economic summary of experiences.

And Einstein's "objective reality" takes us beyond the nonsense of many current quantum ideas, such as that nothing happens until a physicist make a measurement, at which point the universe splits, doubling its mass in extreme violation of the conservation laws that are the basis for Einstein's beliefs.

Einstein's Method

Lehner builds on philosopher of science Arthur Fine's view that Einstein is committed to a method, that Einstein's realism is not an epistemological claim about the relation between science and a mind-independent reality, but rather a "methodological" claim about science and its apparatus." Lehner calls this "methodological realism."

Einstein himself describes his method in his 1933 Herbert Spencer Lecture "On the Method of Theoretical Physics." Science is built on fundamental general "principles" rather than constructed on particular "facts." Einstein's principles are often invariances, like the constant speed of light in all inertial frames and the relativity principle that laws of physics are the same in all such frames, or the principle of covariance in his general relativity.

But well beyond his success in relativity, Einstein's principles include laws of symmetry and the conservation laws that derive from them. Where Niels Bohr spent many years challenging the conservation of energy when light interacts with matter, to preserve his idea that the light is a continuous wave and not Einstein's light quantum (our photon), Einstein based his predictions of how a photon would interact with an electron on the principle of conservation of momentum and energy, following his discovery that light particles have momentum.

Does the molecule receive an impulse when it absorbs or emits the energy ε? For example, let us look at emission from the point of view of classical electrodynamics. When a body emits the
radiation ε it suffers a recoil (momentum) ε/c if the entire amount of radiation energy
is emitted in the same direction. If, however, the emission is a spatially symmetric
process, e.g., a spherical wave, no recoil at all occurs. This alternative also plays a
role in the quantum theory of radiation. When a molecule absorbs or emits the energy
ε in the form of radiation during the transition between quantum theoretically possible
states, then this elementary process can be viewed either as a completely or partially
directed one in space, or also as a symmetrical (nondirected) one. It turns out that we
arrive at a theory that is free of contradictions, only if we interpret those elementary
processes as completely directed processes.

Einstein had in 1916 and 1917 shown that Bohr's continuous and spherically symmetric radiation could not explain the randomly directed emissions of his light quanta. Bohr finally gave in when it was realized that Arthur Holly Compton's discovery of the "Compton Effect" had confirmed Einstein's prediction and invalidated the Bohr-Kramers-Slater claims.

Another of Einstein's basic principles is what he called "Boltzmann's principle" equating entropy with the logarithm of the number of possible arrangements of material particles in phase space, S = k lnW.

What we now designate as "real" in physics is doubtlessly the "spatiotemporally arranged," not the "immediately given." The immediately given can be an illusion. The spatiotemporally arranged can be a sterile concept that doesn't contribute to the elucidation of the connections between the immediately given.

(CPAE,8, Doc. 343)

Lehner says that Einstein is proposing a distinction between "a phenomenal reality as the epistemological basis of our knowledge and a physical reality of spatiotemporally arranged events, which is an intellectual construct."

But as Einstein emphasizes, this "construct" is not built on empirical facts, but on "principles,"which are "free inventions of the human mind" that turn out to fit those facts in our experimental tests.

And Einstein's concern that "events" (point coincidences) in space time are "sterile" is precisely correct. Physics must also explain the behavior of "information structures," in particular their information exchanges (or interactions), which rise to the level of information "communications" between living things, the proper basis for the science of biology.

Information (e.g., one of Boltzmann's arrangements of molecules) is neither matter nor energy, though it needs matter for its embodiment and energy for its communication (or interaction).

Lehner's focus on space-time events reflects the concern of mathematical physicists who specialize in general relativity that "physical reality" consists of tenseless four-dimensional structures that describe all "past" and "future" events existing objectively the same as "present" events. This is John McTaggart's concept of an "A-series" and "B-series" of time.

In McTaggart's "B-series" all events have unchanging descriptions. It eliminates change, progress, and evolution. It was a "timeless" view, or as J. J. C. Smart called it, a "tenseless" view. It was influenced by the Einstein-Minkowski "block universe" in which events in the future are already out there in the four-dimensional space-time,

McTaggart argued that the A series is a necessary component of any full theory of time, since change only occurs in the "A series." But he said that the "A-series" is self-contradictory and that our perception of time is, therefore, ultimately an incoherent illusion.

A "tenseless" universe is the epitome of a deterministic universe, a view favored by most mathematical physicists and many philosophers of science.

Wave-particle duality

Lehner cites H. A. Lorentz's description of Einstein's tentative view, somewhat before Louis de Broglie's "matter waves"( which de Broglie credited to Einstein). In this view, light quanta are point particles whose motion is determined (statistically, of course) by a guiding wave or field (Führungsfeld). This had been Einstein's relation of wave to particle since at least 1909.

Lehner gives an extended discussion of Einstein's short extemporaneous presentation at the blackboard during the fifth Solvay conference. And several more pages are devoted to the fruitless debate with Bohr about the Einstein-Podolsky-Rosen paper and the "completeness" of quantum mechanics.

Lehner then discusses the new relevance of EPR in the light of John Bell's proposed experiments to test for the existence of (what Bell thought was) Einstein's idea of "additional variables", later called "hidden variables" by David Bohm. Bell's logic impressed many, including Lehner, but his physics is questionable.

Bell's work was presented as a theorem about measurable differences in the angular dependence of correlations between distant observers of spin or polarization components of entangled particles. He presented the theorem as a "inequalities" that would appear at certain angles. Young experimenters inspired by the charismatic Bell were very excited that their measurements might prove quantum physics wrong and (at least Bell's version of) Einstein's deterministic theory true. This was Bell's hope of course.

When the experiments confirmed quantum mechanics, Bell said sadly "Einstein's program fails." But it was only Bell's idea of Einstein's program that failed.

In the years since Bell derived his inequalities a large number of physicists work on the "foundations of physics," most hoping for a return to a determinist view, which they mistakenly think Einstein wanted. Einstein never doubted that indeterminism and a statistical interpretation would always be a part of quantum physics. His hopes were for a deeper unified field theory that would somehow underlie quantum physics.

Lehner quotes Einstein's worry that such a field theory might be impossible. He quotes Einstein:

"I consider it entirely possible that physics cannot be founded on the concept of a field, i.e:,
on continuous constructs. Then, nothing will remain of my whole castle in the
air, including the theory of gravitation, [but also the whole rest of contemporary physics."